If you're looking for quality, but aren't looking to break your back with costs (especially if you're looking to upgrade/add-on) G&P is pretty good, and WE Tech has come out with an AEG M4 that doesn't cost much and is almost completely Tokyo Marui compatible. I have one myself and I'm hitting players at 180ft. without issue and my hop-up isn't even cranked all the way up. Search different retailers though; as stated previously, it's more about what your requirements are rather than what is "best". Everyone has a different opinion about what best is. If this is your first M4 and/or are just getting into airsoft, I wouldn't get too excited about trying to go top of the line just yet. Find something that works well and will last you so you can test the waters, and then upgrade to something better/different later on if you feel the need to.
Guardians of Asgaard
"The judge meeting is arranged for terrorist." - G&P Laser Product
Primary - Z-Custom VFC ZTW
LifeLine - TM Glock G18C